I changed my oem air dryer with a rebuilt.(NJT "83 mc9). Before the swap, my pressure was 120 psi max. now it goes to 130 and drops back to 100. is the regulater internal? if not why would it change so? Thanks tom

normally, the governor senses the air pressure via a signal line from the dry tank. it is told when the pressure is at cut-out, cut's out, and sends a signal via a signal line to the air dryer to vent. The air dryer is the last in the chain, so to speak, so should not affect cut out pressure. Air pressure gauge is usually from the dry tank as well. So I have no idea why changing your air dryer changed your cut out pressure, but it's adjustable at the governor. It's not a regulator issue, regulators are used to limit air pressure to a lower amount, like 35 psi for belt tensioners, that sort of thing..

T - your air governor is preset - to change the setting remove the cap - loosen the locking nut adjust the set screw no more than 1/4 turn at a time until you get the desired level then don't forget to reset the lcocking nut and replace cap - HTH

Signal line to the governor runs off the wet tank, and the air gauge was plumbed in the MCI's off the accessory tank way up into the 80's. Lots too many of those on the road and in the hands of busnuts to use the word "usually"...

BW (I hope it's OK to call you that!) I'll grant you that the signal line that tells the governor what the system pressure is could easily come from the wet tank. I never traced the line, and i can't figure out a way to logically parse which tank it would be connected to from tests.

But I have to call you on the air pressure gauge. On my bus at least 1980 MC-5C, the dash gauge is absolutely connected to the dry tank. I was about positive on this, but to be sure I just went out and drained the tanks. Gauge was reading 50 PSI when I started. I drained the wet tank first (it had virtually zero pressure in it), gauge read 50 PSI. I drained the parking brake tank next, gauge read 50 psi, it had significant pressure as you would expect if it's check valve was working. I went around and drained the accessory tank next, moot point because it had zero pressure in it when I popped the valve. Draining the parking brake tank will also drain the accessory tank, but by changing the order and draining the accessory tank first you can verify the operation of the parking brake check valve. Gauge still reads 50 PSI, no change. So my bus is now sitting there with zero pressure in any tank but the dry tank, which I didn't drain, the gauge reads 50 psi, and thankfully the suspension is still up.

For me, this proves two things - the air pressure gauge reads the dry tank, and the check valves on the suspension height controls and the feed line from the wet tank to the dry tank all work. Also, the pressure protection valve is keeping air, to at least the 50 psi that I have in the bus today, inside the dry tank.

Next logical point - when I air up the bus from zero, the gauge immediately starts to read a pressure increase. There is a pressure protection valve between the dry tank and the parking brake/accessory tanks that will not allow air to pass until the pressure exceeds 65 psi. If the gauge was reading the accessory tank, the gauge would read zero for about 5 minutes then rapidly jump to 65 psi, and it doesn't do that. What it does do is climb to 65 psi steadily, then pause for a minute or a bit longer while the two tanks fill, then start to climb again. So that is a way to verify the operation of the pressure protection valve.

What do you think? Is my logic faulty? Maybe I have a funny bus...

edit: realised I should complete the experiment, so I went out and drained the dry tank. Pressure on the gauge dropped to zero.

I'd go back over the plumbing, Something may not be right. Maybe something easy like a piece of dirt in the D2 or maybe it just is failing. Maybe the new drier is defective too, but it sounds like the signal to the D2 is either changing or the D2 is bleeding 30 lbs every time the pressure tops off thru the unloader.

The D2 governor is located on the compressor in most applications.

That could also be a correct action and the governor is just set a little high. The compressor is restarting at 100 and stopping at 130 so it is basically operating correctly, just at a little higher pressures than you had before.

You could also have a tank equalization issue if you haven't pumped the entire system up. Where is the signal for the D2 comming from?

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The signal line needs to come off the first storage place after the compressor, otherwise, a need for air in some isolated part of the system wouldn't summon more air, if the signal was connected deeper, beyond check valves, where it would be blind.

As for the air gauge, viewers, best to take note, I have made a mistake.

Starting with MC8 unit #31843, schematic date April 1978, the air gauge was indeed plumbed to the service tank.Schematic dated December 1976 for units 31513 thru 31842 were plumbed to the accessory.

Prior units in the MC8, and earlier MC7, were plumbed to the accessory tank.

Later units in the MC8 and the MC9 will be plumbed to the service tank.

It would be safe to assume the MC5C were changed concurrently with the MC8, as this would have been tied to the changes in the FMVSS and any exemptions/extensions that were granted, as the switch to dual brake systems and other requirements post 1975 came into effect and expired.

My apologies for the careless post above, and congratulations to Brian for smartening me up.

Brian, does your MC5 manual have the multiple schematics in it, and share the dates?

My 5C manual, as far as this is concerned, has a quite generic schematic that shows none of this detail, except for the location and direction of a couple of the check valves. The governor, the air dryer, the gauge, are all not shown. The date on the schematic is January 1978, Unit 7401.

I found it interesting that by observing the process during air-up and by draining the tanks in a specific order, you can test and verify the action of all of the main check valves in the braking system, and the pressure protection valve, in addition to the normal compressor recovery, low air on and off, compressor cut in and out stuff. I wish I could figure out a way to test the shuttle valve. I think (haven't tried this) that if I chock the bus, turn it off with over 100 psi in the dry tank, drain the dry tank, remove the chocks, roll forward or start it and drive forward and do a test application, that would force the shuttle valve to operate and make the parking brake tank apply the rear service brakes without activating the DD3 parking brake. But if it doesn't work, I then roll down my driveway and into a ditch. So I'm going to think about that for a while before I try it!

I really do think it's critical to be able to test and verify the brake operation, including the failure mode operation, on a regular basis. So I'm going to have to think that through eventually.

That's how to do it. And you should be able to stop by putting the parking brake on, since in theory, there will still be air in the parking tank.

Test it sitting still, observe the movement of the pushrods to confirm operation, and confirm release, and confirm no massive and unstoppable air leak out of the parking tank, then fly on the wild side and do a dynamic test.

My concern has always been that testing these systems is not done for the DOT inspection, and MANY, MANY mechanics have no idea what we are talking about here.

So you don't think that applying the service brakes with the service tank empty will set the parking brake? the way I THINK it's supposed to work is the parking brake tank supplies failure mode air to the front and rear service brakes only, the shuttle disconnects the connection to the inversion valve so the parking brake can't be applied.

Add a judicious amount of gracious to knowledgeable and stir with "ability to communicate" and what do you get?? BW! Thank you so much for not only the info but the inspiring example. Thanks!

Brian,

Great post. I gather that all the tests you performed are not possible in all coaches. From your ability to diagnose failure and verify performance, I think it would be advisable to duplicate your configuration. You say your schematics are not in detail...can that config be detailed in a drawing that I can understand or is it documented somewhere else?

Again, thanks to both of you. Inspiring on many levels.

John

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John, not sure what you are asking. If you like, I can document my air brake testing procedure for you here. it takes a long time do do, I hardly ever do it all at once, to be honest. I can also document what is connected to what tank and how the governor works with the air dryer. I don't know how to draw diagrams with other than a pencil and ruler, though...

I do think it's interesting to work out how to test all of the various bits and bobs in the system, though.

What it was about: I have read a lot of posts over the years from Knuts that had buses that leaked down in short order. I also read about "detailed" schematics for buses and also plans for the entire bus air system. With that in the back of my mind, easily seen from the front, I wondered that so many had problems.....just a little cause I have been stopped in my tracks by some simple stuff myself.

My background is in electronics. In training they introduce you to the "split half method" of troubleshooting. In that you verify performance in the middle and let that decide which way you go. If you get lost in that and can't get any use full info you go "hand over hand" and verify every circuit/component till you find a bad "something". For bus work the troubleshooting always seems to go straight to "hand over hand" and you do the entire bus, front to back. But hey, whatever gets the job done. Still, lots of stuff is nearly impossible to get to without taking off wheels and such so they should be at the bottom of the list.

What you described to me was a way to eliminate the tanks and their associated plumbing as being a problem. You found your wet tank down and I guess that the reed valves in your compressor would be expected to not function as a check valve. In your system, I gather, a leak in any of your accessories would draw down the emergency brake tank that feeds it and then bleed down the rest of the system.

I intuit that the air systems are so dissimilar out there that a general guide to working through a air leak problem can't be done?

Don't feel bad...I can't do that graphics stuff either. I am limited to the tablet and pencil, as well.

John

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And here I was surprised that experienced Knuts were still "thinking through" ways to trouble shoot air systems. And also amazed that there wasn't a documented way to air up and bleed down a system and gain some knowledge of how well the system was performing. All I have ever heard was that to find a leak you went over the system with a bottle of soapy water and watched for bubbles.

From Brian:I found it interesting that by observing the process during air-up and by draining the tanks in a specific order, you can test and verify the action of all of the main check valves (and other stuff) in the braking system, and the pressure protection valve, in addition to the normal compressor recovery, low air on and off, compressor cut in and out stuff.

I really do think it's critical to be able to test and verify the brake operation, including the failure mode operation, on a regular basis. So I'm going to have to think that through eventually.

I think Brian was talking about making a procedure that would do the verification on his peculiar system. And it was pointed out that his gauge might not be connected as "usual" and that that variance would throw off the logic of his observations of airing up or down. I was reaching for : where is the optimum location to connect that gauge into the system? What could be gained in troubleshooting flexability by adding another gauge at what other point?

From Warrior:

My concern has always been that testing these systems is not done for the DOT inspection, and MANY, MANY mechanics have no idea what we are talking about here.

This was an eye opener....I can pay for a DOT inspection of my operating air system to gain assurance that "all is well" and NOT get a comprehensive test. BUT, if I know the system and its sequences, I can do the test myself IF the air pressure gauge(gauges) are ported into the system at certain NON STANDARD points.

I get that there are only so many ways to config a air system but apparently not all sufficiently alike to be able to document a generic procedure. What really caught my eye was the mention of CHECK valves that could leak down the various air subsystems and make nary a bubble that could be detected with that squirt bottle to trouble shoot the cause.

Then there was that brake can issue where there exists a couple versions of brake cans that are definitely not even similar in config. I am certain that in my ignorance there is a lot of other issues. I think the best that could come out of this is if Brian could document his procedure and another Knut could provide or reference the drawings that are relevant.

Another item that caught my eye was that the WET tank bled down in short order. If there is a check valve in the outlet of the compressor I think that proved that that valve wasn't working. Again, the compressor has reed valves that should, in theory, keep the wet tank up. I think that if I had that problem I would be rebuilding my compressor head unless RoadW or Luvr or BK or the likes told me to save my strength. Maybe a check valve in there. Your system used to bleed the suspension down to the ground in an hour or so and it took you a while to air up. Have you found that problem? Was it a leak in the bags or did a check valve fail? I know you live 700 miles from where your 06 is stored and you keep your bats charged with a spendy solar array so she will be ready when you get to her with precious R&R time from your work. I also know that you do the work of 4 men in maintaining home and hearth so God forbid you find crit in anything I said. I have only respect. You have spent the money and sweat and brain power wisely as far as I can see or hear.

Is it possible that that was clear? Warm regards to you and your Lady.

John

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"An uneducated vote is a treasonous act more damaging than any treachery of the battlefield.The price of apathy towards public affairs is to be ruled by evil men." Plato “We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light.”—Pla